The present invention relates to the manufacture of devices for the detection of high-energy electromagnetic radiation and, more particularly, to a method for providing gamma-ray detectors containing cadmium zinc telluride (CdZnTe) crystals with improved properties.
Gamma-ray detectors have wide applications, for example, in medicine, environmental monitoring and materials testing. As an example, in nuclear medicine a gamma-ray-emitting substance, a radiotracer, is typically injected into the body of a patient. The radiotracer travels through or is concentrated in parts of the body. By detecting the gamma rays emitted by the radiotracer using a gamma ray spectrometer, diagnostically useful information is gained.
In FIG. 1 a typical gamma-ray spectrometer 10 is depicted. The heart of gamma-ray spectrometer 10 is gamma-ray detector 11. Gamma-ray detector 11 detects and measures the energy of individual photons, such as photon 18. Gamma-ray detectors, such as 11, typically include a semiconductor crystal 12, a forward contact 14 and a backward contact 16. In detector 11, semiconductor crystal 12 is a CdZnTe crystal. A gamma-ray photon 18 interacting with a semiconductor crystal 12 deposits energy in the bulk of semiconductor crystal 12 by ionization to form electron 20 and hole 22 pairs. Once produced, electrons 20 and holes 22 drift in opposite directions (24 and 26, respectively) under the influence of a strong electric field 28 within which semiconductor crystal 12 is found. The output signal of detector 11 is the induced charge corresponding to charge-motion 24 and 26. Read-out electronics 30, including pulse-shaping amplifier 32 and computer equipped multi-channel analyzer 34, allow a spectral distribution (energy distribution) curve to be recorded. In FIG. 2A, detector 11 is depicted in perspective, showing CdZnTe crystal 12, forward contact 14 and backward contact 16. In FIG. 2B, an embodiment of a detector 11 is depicted, where in addition to CdZnTe crystal 12, forward contact 14 and backward contact 16, is a guard ring 38 maintained at the same potential as forward contact 14 so as to reduce surface leakage current.
The parameters important in defining the usefulness of a gamma ray detector for nuclear medicine are spatial resolution, energy resolution and counting efficiency. In order to minimize the exposure of a patient to radiation, it is preferable that only small amounts of radiotracer be injected. As a result, the absolute intensity of gamma-rays that are emitted by the radiotracer to be detected is low. It is thus important that a gamma-ray detector used in nuclear medicine have a high counting efficiency. In addition, the semiconducting crystal used in a gamma-ray detector must be made of a semi-insulating material. As described above and in FIG. 1, a gamma-ray detector must be located in an intense electric field. The intense electric field increases the level of charge collection. However, the typical intensity of an electric field used is such that large dark currents are induced in materials which are even slightly conductive and are thus not categorized as semi-insulating.
Amongst the suitable semiconducting materials used in gamma ray detectors is crystalline Cadmium Zinc Telluride (CdZnTe). Gamma-ray detectors made using CdZnTe have wide band-gaps, high resistivities and allow room-temperature operation.
Further background of radiation detectors in general and CdZnTe detectors in particular can be found in the prior art, for example in references [1-3]. Greater details concerning the specific background relating to CdZnTe detectors can be found in the prior art, for example in references [4-15].
Known semiconducting materials, including CdZnTe crystals, produced in accordance with the methods known in the art contain a large number of defects. The presence of a large number of defects renders a crystal unsuitable for use in a detector. In the first place, the presence of a large number of defects decreases the resistivity of a crystal, leading to high levels of dark current. In the second place, defects lead to charge-carrier trapping. Charge-carrier trapping occurs when charge-carriers become trapped at defects in the crystal and thus contribute only partially to the signal. When charge-carrier trapping occurs, pulse heights are different for different interaction depths resulting in a tailing or plateauing of a photopeak. This tailing or plateauing reduces the energy resolution of the detector and reduces the effective photopeak fraction. For any given energy window, charge-carrier trapping also reduces the detector counting-efficiency.
It would be highly advantageous to have a method for improving CdZnTe crystals and gamma-ray detectors. It would be highly advantageous to be able to supply CdZnTe crystals having a well-defined and predictable low level of defects so that gamma-ray detectors made using the crystals have a good spectral resolution, high sensitivity and low dark current level.
[1] xe2x80x9cPhysics in Nuclear Medicinexe2x80x9d, J. A. Sorenson and M. E. Phelps, Second Edition, W. B. Saunders Company, London (1987).
[2] xe2x80x9cFoundation of medical imagingxe2x80x9d, Z. H. Cho, J. P. Jones and M. Singh, Wiley, N.Y. (1993).
[3] H. B. Barber xe2x80x9cApplication of II-VI materials to nuclear medicinexe2x80x9d, J. Electronic Materials, 1996, 25, 1232.
[4] Y. Nemirovsky, A. Ruzin, G. Asa, J. Gorelik xe2x80x9cStudy of Charge Collection Efficiency of CdZnTe Radiation Detectorsxe2x80x9d, J. Electronic Materials, 1996, 25, 1221-1231.
[5] Y. Nemirovsky, A. Ruzin, G. Asa, J. Gorelik xe2x80x9cStudy of Contacts to CdZnTe Radiation Detectorsxe2x80x9d, J. Electronic Materials, 1997, 26, 756-764.
[6] A. Ruzin Y. Nemirovsky xe2x80x9cStatistical Models for Charge Collection Efficiency and Variance in Semiconductor Spectrometersxe2x80x9d, J. Appl. Phys., 1997, 82, 2754-2758.
[7] Y. Nemirovsky, G. Asa, C. G. Jakobson, A. Ruzin, J. Gorelik xe2x80x9cDark Noise Currents and Energy Resolution of CdZnTe Spectrometersxe2x80x9d, J. Electronic Materials, 1998, 27, 800-806.
[8] Y. Nemirovsky, G. Asa, A. Ruzin, J. Gorelik, R. Sudharsanan xe2x80x9cCharacterization of Dark Noise in CdZnTe Spectrometersxe2x80x9d, J. Electronic Materials, 1998, 27, 807-813.
[9] A. Ruzin, Y. Nemirovsky xe2x80x9cMethodology for Evaluation of Mobilityxe2x80x94Lifetime Product by Spectroscopy Measurements in CdZnTe Spectrometersxe2x80x9d, J. Appl. Phys., 1997, 82, 4166-4171.
[10] A. Ruzin, Y. Nemirovsky xe2x80x9cPassivation and Surface Leakage in CdZnTe Spectrometersxe2x80x9d, Appl. Phys. Lett., 1997, 71, 2214-2215.
[11] Y. Nemirovsky, G. Gordon, D. Goren xe2x80x9cMeasurement of Band Offsets and Interface Charges by the C-V Matching Methodxe2x80x9d, J. Appl. Phys., 1998, 84, 1-8.
[12]Y. Nemirovsky xe2x80x9cStatistical Modeling of Charge Collection in Semiconductor Gamma-Ray Spectrometersxe2x80x9d, J. Appl. Phys., 1999, 85, 8-15.
[13] Y. Nemirovsky, M. Iframor, A. Ludwig xe2x80x9cThe Effect of the Geometrical Parameters on the Electric Field of Pixilated Two-Dimensional Arrays of Gamma-Ray Spectrometersxe2x80x9d, J. Appl. Phys., 2000, 88, 5388-5394.
[14] Y. Nemirovsky, G. Asa, J. Gorelik, A. Peyser xe2x80x9cRecent Progress in n-Type CdZnTe Arrays for Gamma-Ray Spectroscopyxe2x80x9d, Nuclear Instrument and Methods, A, 2001, 458, 325-333.
[15] M. Ifraimov, A. Ludwig, Y. Nemirovsky xe2x80x9cStatistical Modeling of the Spectral Performance of a Two-Dimensional Array of Gamma-Ray Spectrometersxe2x80x9d to be published in J. Appl. Phys., 2002.
[16] A. P. Zdebskii, N. V. Mironyuk, S. S. Ostapenko, A. U. Savchuk, M. K. Sheinkman Sov. Phys. Semicond., 1986, 20, 1167.
[17] A. P. Zdebskii, M. I. Lisyanskii, N. B. Lukyanchikov Sov. Tech. Phys. Lett., 1987, 13, 550.
[18] G. Garyagdiyev, I. Y. Gorodetskii, B. R. Dzhumayev Sov. Phys. Semicond., 1991, 25, 248.
[19] M. Lisiansky, V. Korchnoi, R. Weil, N. Nemirovsky xe2x80x9cStability of Electrical Parameters of Metalorganic Chemical Vapor Deposition CdTe Layersxe2x80x9d, J. Phys. D, 1997, 30, 3203-3210.
[20] M. L. Lisiansky, V. L. Korchnoi, A. Berner, E. Muranevich, R. Weil xe2x80x9cImprovement of CdTe Substrate Quality by Acoustic Treatmentxe2x80x9d, J. Cryst. Growth, 1999, 197, 630.
[21] F. Edelman, A. Zeckzer, P. Grau, S. Stolyarova, R. Weil, A. Berner, R. Beserman, Y. Nemirovsky xe2x80x9cHardening of Cd1-xZnxTe by Acoustic Wave Treatmentxe2x80x9d, submitted to Physica Status Solidi, 2002.
The invention of the present invention includes a method for the improvement of spectral resolution, sensitivity and uniformity of CdZnTe crystals and CdZnTe gamma-ray detectors by reducing defect density using an acoustic wave treatment.
According to the present invention there is provided a method for producing CdZnTe crystals or gamma-ray detectors having a reduced amount of defects by a) taking a CdZnTe crystal or a gamma-ray detector made of a CdZnTe crystal and b) treating it with acoustic waves. The acoustic waves are of low amplitude in order not to cause the production of defects in the crystal or detector, thus the strain amplitude in the transducer should be in the range 10xe2x88x927 to 10xe2x88x926 m/m. When a CdZnTe crystal is treated according to the method of the present invention, then after the treatment the crystal is integrated into a gamma-ray detector, for example by adding contacts to the crystal according to the methods known in the art.
According to one embodiment of the present invention, the crystal or detector is treated with acoustic waves using a device configured to transfer acoustic waves through a liquid (including liquids, but also suspensions, colloids, gels and the such) and wherein the crystal or detector is immersed in such a liquid.
According to a feature of the present invention, the acoustic waves are applied to a crystal or detector to a face of the crystal, allowing the acoustic waves to propagate perpendicularly to that face.
In order for the acoustic waves to be applied through a face of the treated crystal or detector, it is preferred that the treated crystal or detector be in physical contact with the transducer used to produce the acoustic waves.
According to a feature of the present invention, the crystal or detector is reversibly attached through a face to the transducer. According to a further feature of the present invention, the crystal or detector is reversibly attached to the transducer by means of a removable adhesive such as paraffin or photoresist.
The acoustic waves used are in the range of frequencies between 100 Hz and 10 MHz, preferably ultrasonic acoustic waves. According to a feature of the present invention, it is preferable from the perspective of improved process-control to select a frequency of acoustic waves, which is substantially similar to a resonance frequency of the specific acoustic transducer used. According to a feature of the present invention, a resonance frequency of the transducer-crystal combination (or transducer-detector combination) is determined and that resonance frequency is used according to the method of the present invention.
When ultrasonic waves are used, either shear, compressional or a combination of both waves are used.
According to a feature of the present invention, the crystal or detector can be treated using the acoustic waves at room temperature. According to a still further feature of the present invention the treatment occurs at an elevated temperature of between 20xc2x0 C. and 80xc2x0 C. While not wishing to be held to any one theory, it is believed that an elevated temperature can increase the rate of processes responsible for the positive effects of the method of the present invention. It is also believed that an elevated temperature can supply the activation energy necessary to initiate some processes responsible for the positive effects of the method of the present invention.
According to a feature of the present invention, the treatment of the crystal or detector can advantageously occur in the presence of a DC electric field. Those crystal defects carrying a charge are transported from the crystal more quickly than otherwise due to the effect of the DC electric field. According to a feature of the present invention, such an electric field preferable has an intensity of greater than 50 V/cm and of less than 500 V/cm.
The defects and impurities that are removed from the crystal or detector by the method of the present invention are often driven to the outer surfaces of the crystal or detector. Thus, according to a feature of the present invention, after application of the acoustic waves to a crystal or detector, the surfaces of the crystal or detector are treated to remove the defects from the surfaces. One such treatment is etching of the surfaces using a methanolic solution of Br2.
According to a feature of the present invention, after application of the acoustic waves, the crystal or detector is annealed at an elevated temperature of between 30xc2x0 C. and 90xc2x0 C. According to a still further feature of the present invention annealing is performed for more than 30 minutes. According to a still further feature of the present invention, annealing is performed for no longer than 2 months.